A vehicle (e.g., an automobile) has a filter, which filters foreign objects contained in fuel (see, for example, JP2009-257147A). Particularly, in a case where the fuel is light oil, it is required to appropriately replace the filter with a new filter. In order to meet such a need, it is common to use a filter cartridge, which is formed by supporting a paper filter in a resin support member, and this filter cartridge is appropriately replaced.
A filter mesh size (also referred to as a pore size) of this type of filter is set to fall in a predetermined appropriate range to achieve a desired filtering performance and thereby to remove foreign objects that have a size equal to or larger than a predetermined size. It is conceivable that the filter cartridge having the filter mesh size, which is out of the appropriate range, is used. For instance, when the filter cartridge is replaced with a new filter cartridge having a large filter mesh size, which is larger than the appropriate range, components, such as a high pressure fuel pump and fuel injection valves, which are located on a downstream side of the filter cartridge, may possibly be malfunctioned by the foreign objects, which have passed through the filter. Furthermore, in the case where the components are malfunctioned due to the use of such a filter cartridge, it is difficult to identify the filter cartridge as the cause of the malfunction.
In contrast, when the filter cartridge is replaced with a new filter cartridge having a small filter mesh size, which is smaller than the appropriate range, a pressure loss at the filter may possibly become excessively large, thereby possibly disabling injection of fuel at a desired pressure.
Furthermore, even in the case where the filter cartridge having the appropriate filter mesh size, which falls in the appropriate range, is used, when such a filter cartridge having the appropriate filter mesh size is used for a long period of time or when the preexisting filter cartridge is replaced with a used filter cartridge (a secondhand filter cartridge) having the appropriate filter mesh size, it may be the same as the state where the clogged filter cartridge is used. Therefore, in such a case, the malfunction of the components discussed above may possibly occur.
As discussed above, in the case where the filter cartridge having the filter mesh size falling out of the appropriate range, or the clogged filter cartridge is used, i.e., the filter cartridge does not have the desired filtering performance, the above disadvantage may possibly occur.
In view of the above disadvantages, it has been demanded to enable an appropriate diagnosis of whether the filter cartridge of the vehicle has the desired filtering performance, thereby limiting occurrence of the malfunction of the components discussed above.